Of great contemporary concern in the U.S., as well as in other countries, is the question of what constitutes a healthful dietary intake of the B-vitamin, folate. Although adequate amounts of folate intake protect against colorectal cancer, accumulating evidence indicates that overly abundant quantities among those who harbor pre-cancerous or cancerous lesions may produce a paradoxical acceleration of tumorigenesis. There are many sources of folic acid in the american diet that supplement the quantities naturally present in foods: voluntarily-fortified foods, vitamin supplements and mandatory fortification. The seminal question is whether these sources are collectively sufficient to produce the cancer-promoting effect of folate; a grave concern for which there is already supportive evidence from several pre-clinical and clinical studies. Due to ethical and chronological constraints, this dilemma cannot be resolved by human studies alone. Consistent with this reality are the conclusions of a recent international conference on the topic, which included among the high priority recommendations the need to pursue the issue in appropriate animal models. The overall goal of this proposal, therefore, is to answer these questions within the context of a highly innovative and powerful animal model. This novel animal model of colorectal carcinogenesis, GEM-1, allows one to monitor development of colonic neoplasms in vivo over a period of several months. This will enable us to determine critical parameters such as the threshold of dose at which dietary folate has a promotional effect and to develop a mathematical model that defines the relationship. Our proposed studies will: 1) characterize the GEM-1 model's response to supraphysiological quantities of dietary folic acid in regard to cancer promotion, 2) define the quantitative nature of the dose-response relationship between supraphysiological folic acid intake and acceleration of tumorigenesis, 3) determine whether delivery of supraphysiological quantities of a natural form of the vitamin, 5-methyltetrahydrofolate, lacks the cancer-promoting effects observed with the synthetic form, folic acid, and 4) provide evidence that the molecular pathway by which excess folic acid is driving carcinogenesis is by providing abundant quantities of the rate-limiting substrate for DNA synthesis, thymidine. PUBLIC HEALTH RELEVANCE: There is great debate in the U.S. and in other countries as to what constitutes a healthful dietary intake of the B-vitamin, folate, because many sources of the vitamin have been introduced into the food stream and there is accumulating evidence to suggest that overly abundant quantities of the vitamin may promote the risk of colorectal cancer. The existing data concerning the possible promotion of cancer is rather equivocal, and yet the potential ramifications are very great, making this a particularly pressing public health issue that needs to be addressed with some urgency. The studies proposed in this application will be conducted in a novel and particularly well-suited animal model and will provide important answers as to whether the intake of folate that is helping optimize health for one segment of the population might be presenting potential risks to other segments of the society.